"You give them challenges that somebody of that Skill level can solve, and see when they stop being able to solve them.  For spells, that usually involves casting standard spells by recitation, which, well, you see the problem in your case.  The Adventurer's Guild uses a small blood ritual to find out what common Skills you have when you sign up, so they can check for skills like thievery and assassination.  Though if God has trouble reading your soul... it's not impossible that ritual wouldn't work on you either."

"You give them challenges that somebody of that Skill level can solve, and see when they stop being able to solve them.  For spells, that usually involves casting standard spells by recitation, which, well, you see the problem in your case.  The Adventurer's Guild uses a small blood ritual to find out what common Skills you have when you sign up, so they can check about Skills for thievery and assassination.  Though if God has trouble reading your soul... it's not impossible that ritual wouldn't work on you either."